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Keihin mixture setting - hot weather


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Prior to the 30c+ hot weather we've had recently my bike was running perfectly. Carburation was spot on.

The last 2 trials have been in very hot weather and on tcikover the bike has started hunting a bit like it's running low on fuel. If I rev it then it's ok again for a few seconds then it starts picking and hunting again. The last trial I went to adjust the air mixture screw and found it was only half a turn out (I've not touched it in the 6 years I've had the bike, a 2014 300 Racing). Screwing it right the way in helped but not enough to completely eliminate the issue.

What else can I do? Is there potentially a blockage somewhere? Do they do different air mixture screws?

I don't particularly want to change needles or jets given that it runs great in 'normal' South East UK weather but if i have to switch to a hot weather setting then i will.


Edited by smarty156
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  • smarty156 changed the title to Keihin mixture setting - hot weather

I would start with a carb clean, or at least dump and clean the bowl. With warmer weather I would expect to be unscrewing the air screw for more air if anything, not the other way around. Last month I had an idling issue, promptly fond some slimy stuff in the Keihin float bowl. I'm a little unsure of fuel quality these days...ethanol, additives... A different pilot jet would be the equivalent of a different air screw.

Edited by Glayne
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I've already cleaned the float bowl out and only use 99 RON fuel with zero ethanol. Carb hasn't been completely off and cleaned out though so might start with that and see how I get on. Will get a can of compressed air and make sure I blow everything through too.

It's just odd as I've literally never touched the mixture screw or needles/jets and it's been spot on until the last 2 trials, both of which were circa 30c which is obviously a lot hotter than normal.

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I had a similar issue on the Sherco a fortnight/week ago when practicing. Assumed maybe the fuel was turning, especially due to heat.

Cleaned carb twice, air filter, spark plug, even gearbox oil to make sure everything was smooth. Fresh fuel. Still four-stroking a bit.


In the end I had to adjust the air mix on the carb and the idle until it was a bit more stable. 

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  • 1 month later...

Ok so I still have this issue even though I've had the carb off and cleaned it out (it was perfectly clean though) and temperatures are back to normal (was around 18c today). Spark plug also looks ok and I've put a fresh air filter in.

Again, screwing air screw right in helps a bit but if you leave it idling it starts to hunt and tries to die. If I give it a rev it then sits ok for a while. However, it also seems to be smokey, especially when cold.

Any ideas what the issue could be? As I say, in the 6 years I've owned the bike the carburation has always been spot on and the carb is clean so it's not that. Was wondering if the fuel could be a problem but that's unlikely. Anything else that could cause this?


Edited by smarty156
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  • 2 weeks later...

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